Many erasable inks have been developed over the years. Such inks are usually only suitable for use in ballpoint pens with pressurized ink reservoirs. Characteristics that are beneficial for such use are not practically extendable to writing instruments that have porous tips for ink distribution.
In addition, many of these inks are only initially erasable, becoming essentially permanent within a short period of time. Furthermore, some of these inks can only be used on nonporous writing substrates.
U.S. Pat. No. 4,297,260 discloses an erasable writing medium composition suitable for use in porous-tipped and ballpoint pens. The medium comprises a carboxylated styrene-butadiene latex emulsion and a basic dye. The medium may additionally contain an anti-drying agent and a plasticizer.
U.S. Pat. No. 4,407,985 discloses an ink of 1.5 million cps viscosity suitable for use in ballpoint pens. The ink comprises a pigment, a polar elastomer, and a solvent system comprising a volatile component and an essentially non-volatile component including a pigment vehicle and a low viscosity plasticizer. The polar elastomer may be butadiene/acrylonitrile copolymer rubbers, vinyl acetate/ethylene copolymer rubbers or mixtures thereof. The ink is said to be characterized by an initial erasability with ordinary pencil erasers and by a capability of developingpermanence over a period of time.
U.S. Pat. No. 4,410,643 discloses an ink composition that comprises a pre-pigmented elastomer and a solvent system that contains volatile and nonvolatile components. The ink purportedly has the same erasability characteristics as that of the aforementioned '985 patent and is suitable for use in ballpoint pens.
U.S. Pat. Nos. 4,389,499, 4,390,646 and 4,391,927 pertain to an erasable ink that comprises a pigmented organic solvent solution containing a mixture of thermoplastic block copolymers, such as styrene, ester, or urethane in combination with butadiene, isoprene, or ethylene-butylene. Polybutylene and poly-alpha-methylstyrene are added to decrease smearing. The erasable ink is said to require little or no pressurization and to be suitable for use in ballpoint pens.
U.S. Pat. No. 4,606,769 discloses an erasable ink composition that comprises rubber, a low-boiling aliphatic or aromatic hydrocarbon rubber solvent, a black pigment and a quality adjuster. The composition may additionally contain a surfactant lubricant.
U.S. Pat. Nos. 4,687,791 and 4,760,104 disclose an erasable ballpoint pen ink that comprises a cis-1,4-polyisoprene synthetic rubber or a styrene-butadiene copolymer, volatile and nonvolatile solvents, and a pigment. The composition may additionally comprise fine powders of silicon dioxide.
U.S. Pat. No. 4,721,739 pertains to an erasable ink composition that comprises a styrene-based thermoplastic elastomer block copolymer, a plasticizer, a solvent, a colorant, a resin, a lubricant, and an alkylated polyvinyl pyrrolidone or nonionic surfactant-dispersant.
U.S. Pat. No. 4,738,725 pertains to an initially erasable ink for use in a pressurized ball point pen. The ink comprises an elastomeric polymer, volatile and nonvolatile solvents, and pigment. The elastomeric polymer is selected from thermoplastic and rubbery blocks in which the thermoplastic blocks are selected from the class consisting of styrene and urethane, while the rubbery blocks are selected from butadiene, isoprene, and ethylenebutylene blocks, polyisobutylene polymers, copolymers of ethylene and propylene, and terpolymers of ethylene, propylene, and an unconjugated diene.
U.S. Pat. Nos. 4,889,877 and 4,940,739 pertain to a high-solids content, aqueous, microcapsule-containing printing ink, which comprises an aqueous solution containing water, a nonvolatile diluent, and oil-containing microcapsules dispersed in the aqueous solution. The ink also includes a protective colloid blend of materials selected from styrene-maleic anhydride copolymers, methyl vinyl ether-maleic anhydride copolymers, styrene-maleic ester copolymers, methyl vinyl ether-maleic ester copolymers, carboxymethyl cellulose, casein, polyvinyl alcohol, polyvinyl pyrrolidone, acrylic resins, and styrenated acrylic resins, and may further contain a latex binder emulsion or an alkali-soluble resin.
U.S. Pat. No. 4,940,628 discloses an erasable system including a marking element and an erasable marking composition. The erasable marking composition consists essentially of an aqueous dispersion of a particulate, polymeric film-forming material, a colorant, and a polymeric shear-thinning material.
While the prior art discloses a variety of erasable marking compositions, there remains a need for an erasable marking composition that can be utilized in porous-tipped writing instruments, such as felt tipped markers, as well as with other delivery means, and which can be easily erased with a substantially non-abrasive eraser, such as a common pencil eraser, for an extended period of time.